In production of solar cells based on conventional multicrystalline silicon, the silicon wafers are subjected to a number of heatment steps before being made into final solar cells. One of these heat treatment steps is a diffusion/gettering process comprising a diffusion process where an applied phosphorous source is forced a few micrometers into the surface of the wafer by diffusion in order to create a pn-junction in the surface of the wafer. The source of phosphorous can be gaseous POCl3 or P2C5 dissolved in an organic solvent with addition of for instance SiO2. After vaporation of possible solvents the diffusion of phosphorous into the surface of the wafer is done by heat treatment. The diffusion process is relatively fast and can typically take place at 900° C. in the course of a few minutes. The temperature and the time is chosen in accordance to the electrical properties one wants to achieve in the surface of the water. Thereafter the phosphorous gettering takes place where unwanted dissolved and mobile metallic impurity elements are transported to and captured by the phosphorous layer that has been diffused into the surface of the wafer. This process is typically carried out at 600-850° C. in the course of 1 to 2 hours. This bulk passivation is known as phosphorous gettering or P-gettering.
The lifetime of the minority carriers in a wafer is defined as the time it takes from an electron and a hole is generated from the sunlight intill they are recombined. The lifetime is usually measured in microseconds. If the lifetime for the minority carriers is to short in order that they can move to the pn-junction for the wafer, they will not contribute to production of electric current in the solar cell. The lifetime is reduced among other things by dissolved metal impurities such as Fe, Zn, Ni and Cu in the wafer. It is therefore important for the capability of the solar cell to produce electric current that the amount of dissolved metal impurities can be reduced. It is believed that for instance Fe in conventional multicrystalline silicon wafers is present both as dissolved Fe and as FeSi2-phases. FeSi2-phases will during heat treatment dissolve and increase the content of dissolved Fe in the wafer and cause a reduced lifetime. The gettering process will remove a part of the dissolved iron and other metal impurities, but if the gettering rate for dissolved Fe is lower than the rate by which FeSi2 is dissolved, the netto concentration of dissolved iron will increase and the lifetime of the solar cells will be reduced.
When diffusion/phosphorous gettering at high temperatures (>900° C.) are performed on conventional multicrystalline wafer it has been found that this usually results in a reduction of lifetime for the minority carriers in the wafer and thus is an increased production cost. For this reason conventional multicrystalline solar cells are today produced by the use of moderate temperatures in the range of 600 to 850° C. in order to prevent that the amount of dissolved metals does not become too high. In conventional multicrystalline wafers it is therefore normally not possible to take advantage of high temperature diffusion and phosphorous gettering at for instance 950° C.
In the paper by W. Jooss et. al., “Large Area Buried Contact Solar Cells and Multicrystalline Silicon with Mechanical Surface Texturation and Bulk Passivating”, Proceedings of the 16th European Photovoltaic Solar Energy Conference, 1-5 May 2000, page 1169-1172, it is disclosed that solar cells with buried contacts can be made from conventional multicrystalline silicon wafers where phosphorous gettering is carried out at a temperature up to 950° C. Of the two types of multicrystalline wafers used, identified as wafers from Eurosolare and Bayer, it is stated that problems occurred during processing of wafers from Eurosolare. The highest bulk diffusion lengths which were obtained according to the paper by Jooss were La=180 μm for wafers from Bayer and La=195 μm for wafers from Eurosolare. This corresponds to lifetimes of respectively 27 μs and 36 μs which is relatively low and lower than normal lifetimes for conventional multicrystalline wafers where phosphorous gettering has been carried out using conventional temperatures between 600 and 850° C. It is therefore reason to believe that high temperature phosphorous gettering of these conventional multicrystalline wafers does not result in an increased lifetime for the wafers. Even it in the paper by Jooss et. al. is described that solar cells made from conventional multicrystalline wafers can be manufactured with buried contacts, it is not obtained wafers with increased lifetime compared to conventional multicrystalline wafers where phosphorous gettering has been done at low temperatures. This is also confirmed by the cell efficiencies for the solar cells that are stated in the paper. Efficiencies of 15,9% and 15,6% and higher are normally also achieved for solar cells manufactured from conventional multicrystalline wafers where phosphorous gettering has been carried out at a temperature between 600 and 850° C. and where the solar cells consequently do not have buried contacts.
One of the steps in the manufacturing of solar cells with buried contacts comprises high temperature diffusion in the areas where the buried contacts shall be made. Phosphorous is applied in grooves in the wafer and are diffused into the surface at a typical temperature of 950° C. in a time period of 30 minutes. By buried contacts it is understood electrical contacts buried in grooves in the wafer. This has the big advantage that the part of the surface area of the wafer which is available for energy production is increased compared to wafers where the contacts are situated on the wafer surface.
Monocrystalline wafers are of higher purity than multicrystalline wafers and the lack of grain boundaries results in the above mentioned heat treatment steps does not affect monocrystalline wafers to the same extent than conventional multicrystalline wafers. This makes it possible to use new and more effective solar cells concepts such as for instance making buried contacts, which require heat treatment steps at higher temperatures than what is typical for multicrystalline wafers. Monocrystalline wafers are, however, substantially more costly than multicrystalline wafers.
Conventional multicrystalline wafers are made from electronic grad silicon (EG-Si) and silicon rejects from the electronic industry. This quality of silicon has a very high purity, particularly when it comes to phosphorous and boron. Phosphorous and boron content in this quality of silicon are in practice negligible. When wafers are produced from this material it is first produced an ingot by directional solidification whereafter the ingot is sliced into wafers. During the ingot production the silicon is doped with boron or phosphorous in order to produce p-type material or n-type material. When doping with one of the two doping agents it is assumed that the content of the other is negligible. With a few exceptions multicrystalline solar cells are today made from boron doped material.
In the latest years it has been developed so-called compensated multicrystalline silicon for solar cells. This is silicon which contain both phosphorous and boron and which normally has a higher content of other impurity elements, particularly iron than electronic grade silicon. Compensated multicrystalline silicon is produced by refining, cleaning and directional solidification of metallurgical silicon such as described in WO 2005/063621. Wafers made from compensated multicrystalline silicon thus contain both boron and phosphorous and optionally arsenic and other elements like iron distributed in the bulk material, mainly concentrated in the grain boundaries. Wafers manufactured from compensated multicrystalline silicon based on metallurgical silicon by phosphorous gettering at the same temperatures that are used for phosphorous gettering of conventional multicrystalline wafers have reasonable lifetimes, but normally the lifetime is somewhat lower than the lifetime for wafers produced from conventional multicrystalline wafers.
It is therefore a need for silicon wafers made from compensated multicrystalline silicon with increased lifetime and where solar cells with buried contacts can be made from this material.